1. Field of the Invention
The present invention relates to laser scanning devices, and, more particularly, to the optical systems in color laser printers.
2. Description of the Related Art
Optical systems used in laser printers may be characterized as having three sub-systems, namely, a pre-scan optical sub-system, a scanning sub-system, and a post-scan sub-system, each with appropriate mounting hardware for properly positioning the components thereof. Typically, the pre-scan optical sub-system includes a laser diode with large beam divergence serving as a light source, a collimator lens, and a pre-scan lens. The collimator lens produces a collimated beam from the light emitted by the laser diode. The pre-scan lens focuses the processed beam to a waist.
The scanning sub-system is essentially a motor driven, rotatable, polygonal reflector, having adjacent peripheral mirror surfaces, or facets, that rotate during operation of the printer. The mirror surfaces reflect the collimated and focused beam from the pre-scan optical sub-system. The rotation direction of the reflector determines the scan direction of the beam passing to a scanned object, such as a photosensitive drum in a laser printer. During set-up of the optical system, the pre-scan system components must be aligned properly with the polygonal reflector, and at the proper angle, for proper beam reflectance by the polygonal reflector.
A known post-scan optical system includes lenses which function to transform the light beam reflected from the polygonal reflector of the scanning sub-system into a beam having spot size suitable for the laser printing operation, and which function in what is known in the art as an f-theta lens system. The f-theta lens system functions principally to compensate spot positional location on the scanned object as a function of the scanning mirror rotation angle theta, in order to produce a nearly linear change in position on the scanned object for a linear change in angle of rotation of a polygon facet. The post-scan system may include a plurality of f-theta lenses. In addition, this post-scan optical system provides process direction correction to minimize potential facet-to-facet generated process direction jitter. The post-scan optical sub-system may also include one or more folding mirrors to adapt to the geometry of the printer apparatus. During setup of the laser printer, it is necessary to position the fold mirror and to align the f-theta lens or lenses properly with the fold mirror scan image line, so that the scan image beam strikes the photoconductive member at the desired location. When two or more f-theta lenses are used, setup of the printer further requires proper alignment of the f-theta lenses relative to each other. Separate mounting hardware for each, as is known in the past, compounds alignment difficulties, as each contributes tolerance errors to the overall subsystem tolerance error.
The pre-scan optical sub-system defines the light beam axis between the laser diode source and the rotatable polygonal reflector, and establishes the beam diameters and curvature on that axis. Although the optical components used in this sub-system are relatively uncomplicated from a design standpoint, the pre-scan optical sub-system utilizes very short focal length optics of high numeric aperture for reasons of size and efficiency of coupling to the laser diode. As a result, sensitivity to component tolerance and to placement accuracy is very important. Also, the pre-scan optical sub-system is required to produce a beam waist in the cross scan or processing direction, perpendicular to the scan direction, at a precise location relative to the polygonal reflector. Therefore, proper orientation or alignment of the pre-scan system to the scanning subsystem is important.
Color laser printers employ four printing stations, one each for black, magenta, cyan and yellow toners. It is known to provide each as a more or less independent system, so that each printing station has separate pre-scan, scanning and post-scan subsystems. Therefore, each component of the three major subsystems of an optical scanning system is provided four times in the printer, once for each printing station.
In the overall cost of a laser printer, the printhead, including the optical system is one of the more expensive components, and the optical system is a substantial part of the cost. The most expensive component of an optical system for a laser printer is the polygon mirror/motor component of the scanning subsystem. In a color printer, where four polygon reflectors are used, this highest cost component is supplied four times.
In printing color images with four scanning beams, for highest quality printing, it is important to minimize the locating error created by each printhead imaging system. Compounding errors from the four printheads can result in an unacceptable image quality. Each scan image must be position accurately with respect to the other scanned images, with errors in positioning referred to as xe2x80x9cregistrationxe2x80x9d errors.
Thus, not only is it necessary to align components of the pre-scan, scanning and post-scan subsystems relative to each other within a printhead optical system, it is also necessary to align one printhead relative to the other printheads, so that the four scan images are properly registered with respect to each other, to produce the highest quality image.
What is needed in the art is an improved optical system for color laser scanning devices which minimizes cost and minimizes registration scan errors, while facilitating assembly and setup of the device to meet optical performance requirements.
The present invention provides a laser imaging system for a color laser imaging device that includes individual pre-scan and post-scan optical assemblies for each printhead. Two printheads share a common scanning assembly so that only two scanning assemblies are required in a four color device. The pre-scan and post-scan assemblies have improved structures for adjustment.
The invention comprises, in one form thereof, a laser scanning color imaging device with first, second, third and fourth pre-scan optical assemblies; first, second, third and fourth post-scan optical assemblies; and first and second scanning assemblies. Each scanning assembly includes a rotatable polygon reflector and an axis about which the polygon reflector rotates. Two of the pre-scan optical assemblies and two of the post-scan optical assemblies are operatively associated with one of the scanning assemblies, and the other two of the pre-scan optical assemblies and the other two of the post-scan optical assemblies are operatively associated with the other of the scanning assemblies. The first, second, third and fourth post-scan assemblies each include first and second f-theta lenses and a frame member holding the f-theta lenses.
The invention comprises, in another form thereof, a post-scanning assembly for a laser scanning device. The post-scanning assembly includes a fold mirror having a scan image line; first and second f-theta lenses; and an adjustable frame. The first and second f-theta lenses are mounted on the frame.
The invention comprises, in a further form thereof, a color laser printer with a first polygon reflector rotatable about a first polygon axis, first and second pre-scan assemblies operatively disposed to direct light beams originated thereby to the first polygon reflector, and first and second post-scan optical assemblies configured and arranged to receive reflected light in the form of first and second scanning beams from the first polygon reflector. The first post-scan optical assembly receives reflected light originating from the first pre-scan assembly, and the second post-scan optical assembly receives reflected light originating from the second pre-scan assembly. A first PC drum is operatively arranged to receive the reflected scanning beam from the first post-scan optical assembly; and a second PC drum is operatively arranged to receive the reflected scanning beam from the second post-scan optical assembly. The first and second post-scan optical assemblies each include a fold mirror having a scan image line, a frame and first and second f-theta lenses attached to the frame.
The invention comprises, in still another form thereof, a pre-scan assembly for a laser scanning imaging device, the device including a scanning system having a polygonal reflector rotatable about an axis. The pre-scan assembly has a first laser diode collimation module for emitting a first beam of light at the polygonal reflector, and a second laser diode collimation module for emitting a second beam of light at the polygonal reflector. The first and second laser collimation modules are mounted on first and second pre-scan plates. At least one of the first and second pre-scan plates is rotatable about the axis of the polygonal reflector.
An advantage of the present invention is providing a color laser scanning device having reduced costs for manufacturing and assembly through the reduction of the number of more costly components required in the device, and the use of identical components in corresponding systems of the printer.
Another advantage is providing a laser scanning device having simplified assembly, set-up and adjustment between various components of the device.
Yet another advantage is providing a laser scanning device having reduced registration errors in the scan direction between the individual color images.